1. Field of the Invention
The present invention relates generally to fiber optic systems. More specifically, the invention relates to a method and apparatus for monitoring a laser signal in high power optical fiber amplifiers and lasers.
2. Description of the Related Art
This section is intended to introduce the reader to various aspects of art, which may be related to various aspects of the present invention that are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present invention. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.
Since its introduction in the 1980's, the use of optical fiber in the communications industry has been increasing. Providing a significantly higher bandwidth than its copper wire counterpart, as well as lower losses and less susceptibility to crosstalk, more phone calls are able to be handled and the calls are clearer, especially when they are over long distances. Today, optical fiber is strung around the globe and serves as a backbone for communications such as ground-line telephones, cell phones, cable TV, and networks, including the Internet.
In the 1990's the development of the erbium-doped fiber amplifier (EDFA) further increased the efficiency of fiber optic communications. The EDFA is an optical amplifier made of a glass fiber doped with the rare earth metal erbium. An optical signal may need to be amplified for a variety of reasons. For example, in long runs of fiber, amplification preserves a signal that has been attenuated through losses occurring along the length of the fiber. Additionally, amplification may be used to enable the signal to operate at higher power levels for high power applications, such as laser printing and etching.
Before the development of the EDFA, amplification of an optical signal involved detecting the optical signal, translating it into an electrical signal, and then amplifying the electrical signal. The amplified electrical signal was then converted back into an optical signal for transmission. While there are still opto-electrical amplifiers in use today, optical amplifiers, such as the EDFA are much more prevalent. Many optical amplifiers make use of the physical properties of rare-earth metals such as neodymium, erbium, and ytterbium, for example. These rare-earth metals are doped into an optical fiber which serves as both the signal path and the gain medium. Readily available and inexpensive pump sources, such as semiconductor laser diodes, provide optical energy having wavelengths near 970 nm or 1480 nm. The optical energy is absorbed by the rare-earth metal ions and places the ions in a higher energy state. The energized rare-earth metals subsequently transfer energy to a signal traveling through the doped fiber.
In standard single-mode optical fiber amplifiers, pump light is often coupled into the same single-mode core through which the signal propagates, by the use of single-mode optical coupler devices. However, in an alternate approach known either as a cladding-pumping fiber amplifier or a double-clad fiber amplifier, the pump light is coupled into a larger region known as the inner cladding that encompasses the core but is much larger and can therefore guide more pump light. Such larger area, higher power pumps are readily available and can be utilized to pump single-mode fiber cores to produce very high output powers.
A tapered fiber bundle may be coupled at each end of a cladding-pumped amplifier fiber to couple optical energy from pump sources into the cladding of the doped amplifying fiber. As described above, the power is subsequently transferred to the signal and thus amplifies the signal. The tapered fiber bundles typically include seven or 19 individual optical fibers, but may be manufactured in alternative arrangements and shapes. In a seven or 19 fiber arrangement, the fibers are arranged in a circular pattern around the signal carrying fiber and assuming all fibers are the same size, all fibers are touching in a symmetrical pattern. The signal carrying fiber is a single mode fiber, while the surrounding fibers of the tapered fiber bundle are multimode fibers. The multimode fibers have a larger cross-sectional core as compared to the core of the single mode fiber. The larger cross-sectional area is advantageous to allow for higher power light to couple into the multimode fiber. In turn, this higher power facilitates the transfer of more energy into the single mode fiber and consequently to the signal.
As alluded to previously, high power amplified optical signals may be useful in a number of applications. For example, in Earth-bound communication systems, the high power optical amplifiers are useful for CATV applications in fiber-to-the-home networks. In such networks, a high power signal can be split into hundreds of even thousands of homes to send high speed internet, voice and other signals cheaply. Additionally, besides the many possible uses in conventional Earth-bound communication systems, the signals may be used in communication systems between Earth and satellites, satellite to satellite communications, deep space communication systems, laser printing, LIDAR sensing systems, detection systems and many machining applications.
The propagation of high power laser light through various propagation media incurs many operational obstacles, however. For example, nonlinear effects such as Raman effects, stimulated Brillouin scattering (“SBS”) and four-wave mixing (“FWM”) are induced and increase along the length of the subsequent signal carrying fiber. Raman effects occur when there are multiple wavelengths of light present in a fiber and the power level is high. In a broad spectrum of short optical pulses Raman scattering shifts the light towards longer wavelengths, thus altering the frequency of the light. SBS occurs as a result of acoustic phonons and can lead to a back reflected signal having a slightly larger wavelength than the signal propagating in a forward direction. These reflections limit the ability of an amplifier to produce high output powers. FWM also occurs when more than one signal is propagating in a fiber. Interaction between the multiple signals creates, among other things, sidebands and modulation instability.
Each of these non-linearities may be more or less prevalent in any given system based on the operating parameters of the system. However, the longer the length of fiber carrying the signals, the more pronounced the non-linearities will be. While it may be desirable to monitor the output of an optical amplifier to ensure proper operation and to control output levels, coupling a monitor into the signal path adds length to the signal path, and consequently additional non-linearities. Therefore, a system and method for monitoring the output signal without increasing the length of the signal path fiber is needed.